1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) scanner and image processor for reconstructing image data based on projection data taken from a patient to be examined from multiple directions.
2. Description of the Related Art
In recent years, display monitors have tended to have higher resolutions, decreased size, and reduced thickness. Also, their prices have declined. These trends have promoted adoption of filmless technology in the field of medical imaging diagnostics. The merits of filmless capabilities are not limited to direct consequences such as cost of film itself and the cost spent for storage space. In the field of medical imaging diagnostics, filmless technology offers the advantages that the diagnostic accuracy is improved.
For example, an X-ray computed tomography (CT) scanner stores projection data collected by multislice scans or helical scans. When diagnostic reading is done, the CT scanner reconstructs tomographic image data from the projection data according to reconstruction conditions specified by a human operator such as slice position, resolution, and image slice thickness and displays images. During diagnostic reading, the operator can observe any desired tomographic image while varying the slice position, resolution, and image slice thickness at will. In this respect, it is expected that the X-ray CT scanner provides much improved diagnostic accuracy compared with film-based reading in which the slice position, resolution, and image slice thickness are fixed since printed images are used.
However, this filmless technology has problems to be solved. An actual diagnostic reading session generally starts with reconstructing an image that covers a wide area with a very large image slice thickness and at a low resolution of about 512×512 pixels, for example. The image is then displayed. A judgment is made on this wide-area image as to whether there is any lesion. If any portion that is a suspicious lesion is discovered, the image slice thickness is reduced. Alternatively, the resolution is enhanced, and the display FOV is reduced, for example. An accurate image of 512×512 pixels is reconstructed. This portion of image and surrounding portions are displayed over the whole monitor screen while maintaining the high resolution. Consequently, the legion can be identified more accurately. Furthermore, depending on the doctor, it is necessary to check the tissue structure of the lesion in detail. In this case, a more accurate image may be reconstructed by reducing the image slice thickness further and setting the resolution to a smaller display FOV, for example, and the image may be displayed.
In this way, lesional areas are narrowed down while increasing the resolution and reducing the image slice thickness gradually. Whenever the resolution and image slice thickness are varied, the reconstruction processing is repeated. The reconstruction processing is one of the most time-consuming processes. Therefore, the efficiency of the diagnostic reading work deteriorates. Especially, this technology is unsuited for screening examinations for many patients.